1. Field of the Disclosure
This disclosure generally relates to exploration for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. More specifically, this disclosure relates to highly localized borehole investigations of multifrequency focusing of survey currents injected into the wall of a borehole by capacitive coupling of electrodes on a tool moved along the borehole with the earth formation.
2. Background of the Art
Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, a measure electrode (current source or sink) are used in conjunction with a diffuse return electrode (such as the tool body). A measure current flows in a circuit that connects a current source to the measure electrode, through the earth formation to the return electrode and back to the current source in the tool. In inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present disclosure belongs to the first category.
There are several modes of operation. In one, the current at the measuring electrode is maintained constant and a voltage is measured, while in the second mode, the voltage of the electrode is fixed and the current flowing from the electrode is measured. Ideally, it is desirable that if the current is varied to maintain at a constant value the voltage between measure and return electrodes, the current is inversely proportional to the resistivity of the earth formation being investigated. Conversely, it is desirable that if this current is maintained constant, the voltage measured between monitor electrodes is proportional to the resistivity of the earth formation being investigated. Ohm's law teaches that if both current and voltage vary, the resistivity of the earth formation is proportional to the ratio of the voltage to the current.
Birdwell (U.S. Pat. No. 3,365,658) teaches the use of a focused electrode for determination of the resistivity of subsurface formations. A survey current is emitted from a central survey electrode into adjacent earth formations. This survey current is focused into a relatively narrow beam of current outwardly from the borehole by use of a focusing current emitted from nearby focusing electrodes located adjacent the survey electrode and on either side thereof. Ajam et al (U.S. Pat. No. 4,122,387) discloses an apparatus wherein simultaneous logs may be made at different lateral distances through a formation from a borehole by guard electrode systems located on a sonde which is lowered into the borehole by a logging cable. A single oscillator controls the frequency of two formation currents flowing through the formation at the desired different lateral depths from the borehole. The armor of the logging cable acts as the current return for one of the guard one of the guard electrode systems, and a cable electrode in a cable electrode assembly immediately above the logging sonde acts as the current return for the second guard electrode system. Two embodiments are also disclosed for measuring reference voltages between electrodes in the cable electrode assembly and the guard electrode systems.
Techniques for investigating the earth formation with arrays of measuring electrodes have been proposed. An exemplary patent uses an array of small electrode buttons either mounted on a tool or a pad and each of which introduces in sequence a separately measurable survey current for an electrical investigation of the earth formation. The electrode buttons are placed in a horizontal plane with circumferential spacings between electrodes and a device for sequentially exciting and measuring a survey current from the electrodes is described. Another patent discloses tool mounted pads, each with a plurality of small measure electrodes from which individually measurable survey currents are injected toward the wall of the borehole. The measure electrodes are arranged in an array in which the measure electrodes are so placed at intervals along at least a circumferential direction (about the borehole axis) as to inject survey currents into the borehole wall segments which overlap with each other to a predetermined extent as the tool is moved along the borehole.
U.S. Pat. No. 6,714,014 to Evans et al, having the same assignee as the present disclosure and the contents of which are incorporated herein by reference, teaches the use of capacitive coupling with the formation through both oil-based mud and water-based mud.
The Dory patent (U.S. Pat. No. 5,502,686) discloses the use of an acoustic sensor in combination with pad mounted electrodes, the use of the acoustic sensors making it possible to fill in the gaps in the image obtained by using pad mounted electrodes due to the fact that in large diameter boreholes, the pads will necessarily not provide a complete coverage of the borehole.
The prior art devices, being contact devices, are sensitive to the effects of borehole rugosity: the currents flowing from the electrodes depend upon good contact between the electrode and the borehole wall. If the borehole wall is irregular, the contact and the current from the electrodes are irregular, resulting in inaccurate imaging of the borehole. A second drawback is the relatively shallow depth of investigation caused by the use of measure electrodes at the same potential as the pad and the resulting divergence of the measure currents. U.S. Pat. No. 6,809,521 to Tabarovsky et al. discloses a multi-frequency method for determination of formation resistivity. The assumption made in Tabarovsky is that
            σ      1              ɛ      1        ⪡  ω  ⪡            σ      2              ɛ      2      where the σ's are conductivities, the ∈'s are dielectric constant, ω is the operating frequency, the subscript 1 refers to the mud and the subscript 2 refers to the formation. The first of the two inequalities is easily satisfied with oil based mud where the mud conductivity is extremely small. However, if the mud has a finite conductivity, the condition is hard to satisfy. It would be desirable to have an apparatus and method of determination of formation resistivity that is relatively insensitive to borehole rugosity and can be used with either water based or with oil-based muds for a wide range of formation resistivities. The present disclosure satisfies this need.
U.S. Pat. No. 7,397,250 to of Bespalov et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference, discloses a dual frequency apparatus and method for borehole resistivity imaging. There are a number of technically challenging issues that still remain. One of these is the elimination of “galvanic” cross-talk between sensor electrodes through non-conductive mud and a conductive formation. This error becomes more pronounced in the presence of borehole rugosity when the sensor experience uneven standoff from the formation. Another problem with multi-electrode imaging tools is the presence of mutual inductive coupling between circuits defined by the individual button electrodes. Most importantly, while prior art methods recognize the need for methods and hardware for maintaining the buttons at equipotential using, for example, focusing electrodes, this still remains a difficult technical problem at elevated frequencies (in the MHz range). In addition, multifrequency methods require that each of the amplifiers be maintained at proper tuning at a plurality of frequencies.